In view of excellent physical properties such as high heat resistance, moisture proof, dimensional stability, etc., epoxy resin compositions each containing an epoxy resin and a curing agent therefor as essential components are widely used for electronic components such as a semiconductor encapsulating material, a printed circuit board, a build-up substrate, and resist ink, a conductive adhesive such as a conductive paste and other adhesives, a liquid sealing material such as an underfill, a liquid crystal sealing material, a cover lay for a flexible substrate, an adhesive film for build up, a coating material, a photoresist material, a color developing material, a fiber-reinforced composite material, and the like.
Among these, particularly, fiber-reinforced resin composite materials produced by impregnating reinforcing fibers with an epoxy resin and a curing agent as matrix components and then curing the resin are highly required in general industrial fields such as automobile industry and aerospace industry from the viewpoint of various excellent performances such as high heat resistance, low curing shrinkage percentage, chemical resistance, high elastic modulus, etc. in addition to properties such as light weight and high strength.
However, epoxy resins are generally high-viscosity fluids or solids at normal temperature, and thus in a step of impregnating fiber reinforcements with the resins, it is necessary to heat resin components to 100° C. or more in order to secure a practical level of fluidity for the epoxy resins, thereby causing the problem of accelerating curing of the epoxy resins by heating and rather bringing about higher viscosity and impregnation failure. In particular, in a resin transfer molding (RTM) process which has recently been being popularized in the field of carbon fiber-reinforced thermosetting plastics (CFRP) because of the overwhelming cycle time and low equipment cost, low viscosity and high fluidity are essential properties for thermosetting resin materials.
Therefore, there has been known a technique for improving CFRP productivity by the RTM process, in which as an epoxy resin material suitable for the RTM process in CFRP application, a bisphenol F epoxy resin having an epoxy equivalent of, for example, 200 g/eq. or less, is used as a base resin, and aromatic polyamine, which is liquid at room temperature, and a complex of a Lewis acid and a base are used as curing agent components, thereby improving fluidity of a thermosetting resin component and further improving low-temperature curability (refer to PTL 1).
However, in the thermosetting resin material containing the bisphenol F epoxy resin having an epoxy equivalent of 200 g/eq. or less, the aromatic polyamine which is liquid at room temperature, and the complex of a Lewis acid and a base, the viscosity of the epoxy resin is decreased, but the viscosity of the whole composition is still high, thereby necessitating heating for resin injection in RTM molding. Therefore, the possibility of thickening by curing reaction remains, and the running cost is increased in terms of energy. In addition, a cured product has unsatisfactory mechanical strength and heat resistance and has difficulty in applying to the automobile industry and the aerospace industry.
Also, there has been known a technique for producing a cured produced having excellent heat resistance by curing a composition prepared by mixing maleic acid monoallyloxy ester with an epoxy resin (refer to PTL 2 below). However, this composition has high viscosity and thus exhibits poor productivity even when being applied to fiber-reinforced resin molded products.